You're expecting the ES6 arrow function () => {} syntax to set the context (this) the same as the old function declaration syntax function () {} would.

From MDN:

Until arrow functions, every new function defined its own this value (a new object in case of a constructor, undefined in strict mode function calls, the context object if the function is called as an "object method", etc.). This proved to be annoying with an object-oriented style of programming.

I don't know a ton about how Vue.js sets/handles context at this point, but it looks like your method is being called from your template/the DOM and the context is being passed from there into your method. Since the arrow function inherits its context, this refers to the window object.